Quote:
Originally Posted by Liz
Does this mean that I shouldnt use 40mm and over in my scope, or does it depend on the other measurements - focal length, eye relief, exit pupil etc.? 
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Hi Liz,
There are a few reasons, some of which you answered.
Eye relief isn't a problem. The longer the better though.
Exit pupil, if this exceeds 7mm, light is being wasted as it all doesn't get though our pupil. And as we age, this exit pupil we should look at reducing to 6.5mm, or even 6mm. A good rule to follow is this:
longest focal length EP = 6 X focal ratio of scope
Then, because EPs vary in focal length, the absolute max limit will be 7 X the focal ratio.
The longer the focal length too, you may start seeing the shadow of the secondary mirror if your scope is a reflector of some type.
The last consideration I can think about, especially for a fast scope, is the quality of the EP and how coma and astigmatism affects the image quality. The faster the scope, the more obvious the coma is. It isn't a manufacturing defect, its physics of light and glass. The better quality EPs will be able to correct for this much better than cheap ones, but also, if the field of view gets into the extreme, even high quality EPs need the extra help of coma correctors, but these won't help with astigmatism.
Astigmatism IS a quality issue, that is why cheaper EPs are cheap, the quality of glass and lens design doesn't work well enough.
Your scope is f/4.7, the max f.l. would be 28.2mm, with a margin of upto 33mm. Any longer and you a cheating yourself of light. It'll still work, but the effect would be like reducing the apeture of the scope- the image won't get any brighter.
I really like the Astro-Tech Titan Type II's range as they are the exact same EP as the TMB Paragons, and a little cheaper. Their quality is fantastic for their price, and the level of coma correction is brilliant, with only a hint of it along the very edge of the FOV. Their FOV is also 68deg., which matches the angular movement of our HUMAN EYES! Not 82deg, or 100deg. But, that is my preference.
As an even cheaper alternative, the GSO Superviews (other than the 20mm), are very good. They do exhibit more coma, and some astigmatism, but if you are prepared to accept these short comings, they are excellent 68deg FOV EPs for their price.
Mental.